The present invention relates to a rubber composition and a tire, particularly a rubber composition which can improve performances of studless tire on icy and snowy road and a tire whose tread comprises the rubber composition.
Heretofore, when automobiles are driven in cold region with heavy snowfall, studded tires or snow chain-equipped tires are used to ensure the safety on snowy and icy road. However, studded tires or chain-mounted tires easily wear out the road surface, generating dust to cause serious air pollution, which is now regarded as a significant environmental problem.
In order to guarantee safety and solve environmental issues mentioned above, studless tires which has appropriate braking and driving performances on snowy and icy road without any stud or chain has been rapidly developed.
As a studless tire, automobile tires whose performance on ice is improved by compounding sand (JP-A-61-150803) or metallic fibers (JP-A-63-34026) into the tread rubber are proposed. However, in such tires, hardness of the rubber becomes relatively high, and therefore friction on ice is insufficient. In addition, as tire wears off, those sand, metallic fibers and other metals fall off and the particles are spattered to cause a serious environmental problem of dust pollution.
Alternatively, it is also proposed to use foamed rubber for the tread rubber (JP-A-62-283001, JP-A-63-9042 and JP-A- 1-118542). However, in these tires, though frictional force on ice is improved, edge effect and water discharging ability due to voids on the tread surface cannot be fully utilized because the foamed rubber has low block stiffness, and abrasion resistance and steering performance on dry road become inferior. Moreover, since foaming is performed in the vulcanization step of the production, dimensional accuracy is likely to be influenced.
In addition, techniques to compound hollow particles in the tread to improve tire performances on icy and snowy road are proposed (JP-A-11-35736 and JP-A-6-328906). However, in these tires, fine hollow particles are broken during kneading, resulting in a problem that sufficient icy and snowy road performances cannot be achieved.
For another technique, it is proposed to compound a water-absorbing synthetic polymer in the tread rubber to remove water existing between the road surface and the tread (JP-A-5-148390). However, the water absorbing ability of such synthetic polymer is insufficient, and it cannot be said that edge effect after the polymer has fallen is sufficient because the falling of the polymer does not take place immediately after the water absorption.
Other than the above, a method of improving grip performance on icy road by compounding staple fibers orientating them in the direction perpendicular to the tread face to increase digging frictional force is proposed (JP-A-2000-168315). According to these techniques, studless tires have acquired improved grip performance on icy road, but the performance is not yet comparable to that of studded tires.